BACKGROUND OF THE INVENTION As current processes and technologies used in connection with the production of semiconductor circuits scale further into deep submicron levels, the reduction in gate oxide thickness requires that the voltages around a circuit have to be reduced as well.

Hence, in recent years, designing important analog building blocks for lower voltage performance has become a primary concern. Furthermore, careful design of these analog building blocks in a lower voltage environment leads to reduced power consumption, which is extremely important in hand-held and mobile devices. Designing with a lower supply voltage for RF building blocks is all the more important, since the RF front-end of mobile devices has always been the culprit in terms of supply voltage and power consumption.

Fig. 1 on the appended drawing illustrates an embodiment of a known RF front-end receiver.

In a manner known per se, the RF front-end receiver in Fig. 1 comprises a low noise amplifier LNA and a local oscillator driver LOD, which are connected to respective input ports of a mixer.

The low noise amplifier LNA receives an RF signal RF and supplies it to one mixer input port and the local oscillator driver LOD receives a local oscillator signal LO and supplies it to the other mixer input port. From the RF signal RF and the local oscillator signal LO, the mixer generates an intermediate frequency signal on its output terminals IF+, IF-.

The mixer is a variant of a standard Gilbert cell mixer and comprises a transconductance stage and a switching stage. By means of mixers of the type shown in Fig. 1, it is possible to use lower supply voltages.

The transconductance stage comprises two transistors M1 and M2. The gates of the transistors M1 and M2 form the mixer input port for the RF signal RF and are coupled to respective output terminals RF+, RF-of the low noise amplifier LNA. The sources of the transistors M1 and M2 are interconnected to ground, and the drains of the transistors M1 and M2 are coupled to respective interconnected sources of switching transistors M3, M4 and M5, M6, respectively, of the mixer switching stage.

In the switching stage of the mixer in Fig. 1, the gates of the transistors M3, M6 and M4, M5, respectively, are interconnected and form the mixer input port for the local oscillator signal, that is coupled to the output terminals of the local oscillator driver LOD.

The drains of the transistors M3, M5 and M4, M6, respectively, are interconnected and form the mixer output port IF+, IF-for the intermediate frequency signal.

The mixer in Fig. 1 will operate in a balanced manner as long as the input RF signal from the low noise amplifier LNA is differential. Since the mixer is not fully differential, it will not provide any common mode rejection. Common mode feedback can be employed at the IF output port, but nonetheless the mixer will have a lower common mode rejection ratio than a standard Gilbert cell mixer. In order to maintain reasonable common mode rejection in the RF front-end, it is important that the LNA that precedes this mixer is fully differential.

SUMMARY OF THE INVENTION The object of the invention is to bring about an RF front-end receiver that can operate at low supply voltages and at the same time provide common mode rejection.

This is generally attained by modifying the way the RF and LO input signals are fed into the mixer.

More specifically, in an RF front-end receiver that comprises a low noise amplifier and a local oscillator driver, which are connected to respective input ports of a mixer that comprises a first and a second transistor having their gates coupled to one input terminal of a first input port of the mixer, a third and a fourth transistor having their gates coupled to the other input terminal of the first input port of the mixer, and a fifth and a sixth transistor having their gates coupled to respective input terminal of a second input port of the mixer, where the sources of the first and third transistors are coupled to the drain of the fifth transistor, the sources of the second and fourth transistor are coupled to the drain of the sixth transistor, the sources of the fifth and sixth transistor are coupled to ground, the drains of the first and fourth transistor are coupled to one output terminal of a mixer output port, and the drains of the second and third transistor are coupled to the other output terminal of the mixer output port, this is attained in that the low noise amplifier is connected with its output terminals to the first input port of the mixer, and the local oscillator driver is connected with its output terminals to the second input port of the mixer.

Thus, the RF input signal is fed to transistors above the transistors to which the LO input signal is fed. Basically, with respect to the RF input signal, the mixer consists of two differential pairs, which are cross-coupled at the output. The advantage of this arrangement of the inputs, where the RF input signal is fed to transistors above the LO input signal, is that although the mixer has only two stacks of transistors, it is still fully differential. Therefore, the mixer can operate at supply voltages similar to the mixer described above with the added bonus of common mode rejection.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing on which Fig. 1, described above, illustrates an embodiment of a known RF

front-end receiver, and Fig. 2 illustrates an embodiment of an RF front-end receiver according to the invention.

DESCRIPTION OF THE INVENTION Fig. 2 illustrates an RF front-end receiver according to the invention.

Elements in Fig. 2, which are identical to elements in Fig. 1 have been provided with the same reference characters.

In accordance with the invention, the switching stage in the embodiment in Fig. 1 is made the transconductance stage in Fig. 2 and the transconductance stage in the embodiment in Fig. 1 is made the switching stage in Fig. 2.

Thus, in Fig. 2, the low noise amplifier LNA is connected with its output terminals to the interconnected gates of the transistors M3, M6 and M4, M5, respectively, that form a mixer input port RF+, RF-for the RF signal.

Moreover, in Fig. 2, the local oscillator driver LOD is coupled with its output terminals to the gates of the transistors M1 and M2 that form a mixer input port LO+, LO-for the local oscillator signal.

As mentioned above, with respect to the RF input signal, the two stack mixer in Fig. 2 is basically two differential pairs, which are cross coupled at the output. Each of these differential pairs is turned ON and OFF in sequence, controlled by the large input signal from the local oscillator LO.

In the LO+ phase, the LO+ input is high while the LO-input is low. As a result transistor M2 is OFF, which in turn switches off the differential pair formed by transistors M5 and M6, while the differential pair formed by transistors M1, M3 and M4 is switched ON.

Although transistor Ml is switched ON by the high LO+ signal at its gate, the circuit is designed so that transistor M1 is held in saturation and not allowed to go into the triode

region. Therefore, transistor Ml can form the tail current source of a differential pair. The differential RF input signal can now give rise to a differential IF output via the gain of the differential pair of M1, M3 and M4.

In the LO-phase, transistor M1 is switched OFF while transistor M2 turns ON. Now, the differential pair is formed by transistors M2, M5 and M6. For this differential pair, the RF input signal is connected with the opposite polarity compared to the previous differential pair in the LO+ phase. Therefore, the polarity of the IF output will also be reversed, although the gain of the system still remains the same since both differential pairs are identical.

For the mixer in Fig. 2 to operate as described above, it is important that transistor Ml be held in saturation while ON. Moreover, the quality of the differential pair strongly influences the gain of the system and the amount of common mode rejection that the system will provide. This represents an important trade-off in the design of transistors Ml and M2. On the one hand, they should be sized with small channel lengths for fast switching and high frequency operation. On the other hand, they are also required to provide a reasonable output resistance (drain-source resistance) when ON in order to form a high quality differential pair.

With careful design of the mixer core, it is possible to obtain reasonable switching times as well as performance and common mode rejection from the differential pair.

The common mode rejection will definitely be better than in the known embodiment shown in Fig. 1. This represents an overall trade-off between supply voltage and common mode rejection.

Another important system level consideration when using this mixer is that the whole differential pair is switching ON and OFF at the RF input. This means that the input impedance of the mixer can change periodically during operation. This condition is eased a bit since while one differential pair is switching ON, another is switching OFF and

careful design and layout can ensure that there is reasonable cancellation of the switching effects at the RF input.

Using low threshold transistors (with threshold voltages of about 0V) for transistors M3- M6 will help reduce the required gate-source voltages of these devices. This is because the required gate-source voltage will be approximately the required gate-source overdrive voltage if the threshold voltage is roughly 0V. Therefore, using low threshold transistors for M3-M6 can help further reduce the supply voltage requirements of the whole receiver by reducing the supply voltage requirements of the LNA coupled to the mixer.

Many newer technologies do offer transistors with various threshold voltages and oxide thicknesses. However, low threshold transistors are not available in all technologies.

Nonetheless, low threshold transistors can be obtained in any CMOS technology by simply masking out the threshold channel implant, thus providing a native MOS transistor with a threshold voltage of around 0V (for an NMOS device). The two stack mixer according to the invention has been implemented in a technology where low threshold devices were not explicitly available. Instead, they were obtained by masking out the threshold implant.

Reducing the threshold voltage of a MOS device can have important implications for the leakage drain current through the device. However, leakage currents will not be a problem with the mixer circuit according to the invention. Although the devices are being switched ON and OFF and hence might have a higher leakage current in the OFF state, the overall output IF current is orders of magnitude higher than the leakage currents.

Hence, any leakage current will not noticeably affect the output.

Furthermore the transistors M1 and M2 coupled to the local oscillator driver LOD are not low threshold devices, and hence they will help limit the overall leakage current that flows down the ground.

As stated above, the mixer can operate at supply voltages similar to the mixer described in connection with Fig. 1 with the added bonus of common mode rejection.